1. Field of the Invention
The present invention relates to a catalyst for purifying exhaust gases and a process for purifying exhaust gases. More particularly, it relates to the catalyst and the process which can efficiently purify nitrogen oxides (NO.sub.x) in the exhaust gases which contain oxygen in excessive amounts required for oxidizing carbon monoxide (CO) and hydrocarbons (HC) therein.
2. Description of Related Art
As catalysts for purifying automotive exhaust gases, there have been employed 3-way catalysts so far which oxidize CO and HC and reduces NO.sub.x simultaneously to purify the exhaust gases. For example, the 3-way catalysts have been known widely which comprise a heat resistant support formed of cordierite, a catalyst carrier layer formed of gamma-alumina and disposed on the support, and a noble metal selected from the group consisting of Pt, Pd and Rh and loaded on the catalyst carrier layer.
The purifying performance of the 3-way catalysts for purifying exhaust gases depends on the air-fuel ratio (A/F) of automotive engine. For instance, when the air-fuel ratio is large, i.e., when the fuel concentration is small (or on the fuel-lean side), the oxygen concentration is large. Accordingly, the oxidation reactions purifying CO and HC are active, but the reduction reactions purifying NO.sub.x are inactive. On the other hand, when the air-fuel ratio is small, i.e., when the fuel concentration is more (or on the fuel-rich side), the oxygen concentration is small. Accordingly, the oxidation reactions are inactive, but the reduction reactions are active.
Moreover, when driving automobiles, especially when driving automobiles in urban areas, the automobiles are accelerated and decelerated frequently. Consequently, the air-fuel ratio varies frequently in the range of from the values adjacent to the stoichiometric point (or the ideal air-fuel ratio) to the fuel-rich side. In order to satisfy the low fuel consumption requirement during the driving in urban areas, it is necessary to operate the automobiles on the fuel-lean side where the air-fuel mixture containing oxygen as excessive as possible is supplied to the engines. Hence, it has been desired to develop a catalyst which is capable of adequately purifying NO.sub.x on the fuel-lean side (i.e., in the oxygen-rich atmospheres).
Under the circumstances, as set forth in Japanese Unexamined Patent Publication (KOKAI) No. 5-168860, the applicants of the present invention disclosed a catalyst with rare-earth elements including La, and Pt loaded. With the catalyst, NO.sub.x is adsorbed on the rare-earth elements during the the fuel-lean side (i.e., in the oxygen-rich atmospheres) driving, and it is purified by reacting with the reducing gases such as HC and the like, when the automobiles are driven under the air-fuel ratios at the stoichiometric point or on the fuel-rich side (i.e., in the oxygen-lean atmospheres). Thus, the catalyst is superb in the purifying performance of NO.sub.x even on the fuel-lean side (i.e., in the oxygen-rich atmospheres). The catalyst is believed to provide the advantageous effect because the rare-earth elements are loaded on the support in a form of the simple oxides and the simple oxides of the rare-earth elements and they react with NO.sub.x so as to produce the nitrates thereof.
However, the exhaust gases usually contain SO.sub.2 which are produced by burning sulfur (S) contained in the fuel. Further, the catalyst ingredient oxidizes SO.sub.2 to SO.sub.3 in the oxygen-rich atmospheres (i.e., on the fuel-lean side). Then, SO.sub.3 reacts readily with water contained in the exhaust gases to produce sulfuric acid. The sulfate ions or the sulfite ions react with rare-earth elements to produce the sulfates or the sulfites incapable of adsorbing NO.sub.x. Accordingly, the catalyst is not completely satisfactory in terms of the NO.sub.x purifying performance and the durability of the performance at elevated temperatures.